JP2713031B2 - Rare earth doped multi-core fiber and optical amplifier using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rare earth element-doped multi-core fiber in which a plurality of cores containing a rare earth element such as Er and Na are provided in a cladding having a low refractive index and an optical amplifier using the same.

[0002]

2. Description of the Related Art In recent years, rare earth elements such as Er (erbium), Nd (neodymium), and Pr (praseodymium) have been added to the core of an optical fiber, and the optical fiber has an absorption characteristic inherent to the added rare earth element. Research and development of optical fiber amplifiers that amplify signal light by pumping the pumping light have been activated.

FIG. 12 shows a configuration example of a conventional optical fiber amplifier.

[0004] This is because the signal light in the 1.5 μm wavelength band propagates through the core of the optical fiber 22 doped with Er as shown by arrows 20 and 21, and the wavelength of the signal light passes through the optical directional coupler 23 from the middle. 1.47 μm (or 0.98 μm
m) is driven by the drive circuit 25 by the drive circuit 25, and the pump light is also propagated through the optical fiber 22, thereby realizing a population inversion state, whereby the signal light is increased several hundred times to 10,000 times. It has the effect of amplifying to the extent. The optical directional coupler 23 on the output side has a function for removing the light of the pumping semiconductor laser included in the amplified signal light.

[0005]

However, the conventional optical fiber amplifier has the following problems to be solved.

(1) The power of the signal light entering the core is +1
When it becomes 0 dBm or more, the amplification degree sharply decreases. That is, it is difficult to obtain a large optical power on the output side.

(2) Therefore, it is difficult to realize a so-called distribution system for distributing signal light to several tens or more.

(3) The pumping light that has entered the core does not efficiently contribute to the amplifier, and considerable pumping light is emitted from the output side directional coupler, which is uneconomical.

Accordingly, an object of the present invention is to solve the above-mentioned drawbacks of the prior art, perform power amplification, and consequently realize a power transmission and distribution transmission system, and use a rare earth-doped multi-core fiber that can efficiently use pump light. It is to provide an optical amplifier using the same.

[0010]

In order to achieve the above object, the present invention uses a rare earth element-doped multi-core fiber in which a plurality of high refractive index cores containing a rare earth element are provided in a low refractive index cladding. The value of the added amount and refractive index of the rare earth element in each of the cores, and further, the types of the rare earth elements are different, and a low refractive index layer lower than the refractive index of the clad is provided on the outer periphery of the core group, It is still another object of the present invention to provide an optical fiber amplifier that transmits pump light in the fiber.

That is, in the first invention, a plurality of high-refractive-index cores containing a rare-earth element are provided in a low-refractive-index cladding, and at least one of the cores has a rare-earth element-added amount. It is a rare earth element-doped multi-core fiber characterized by being different.

[0012] A second aspect of the present invention is that a clad having a low refractive index has
A rare-earth element-doped multi-core fiber characterized in that a plurality of cores having a high refractive index containing a rare earth element are provided, and at least one of the cores has a different refractive index value.

[0013] A third aspect of the present invention is that a clad having a low refractive index has
A plurality of high refractive index cores containing rare earth elements are concentrated and distributed in the vicinity of the center of the clad, and a ring-shaped low core having a value lower than the refractive index of the clad is formed around the area where the core is concentrated. This is a rare earth element-doped multi-core fiber provided with a refractive index layer.

According to a fourth aspect, in the third aspect, there is provided the rare-earth element-doped multi-core fiber, wherein at least one of the cores has a different amount of the rare-earth element added.

A fifth aspect of the present invention is the rare-earth element-doped multi-core fiber according to the third aspect, wherein at least one of the cores has a different refractive index value.

According to a sixth aspect of the present invention, in the clad having a low refractive index,
A rare earth element-doped multi-core fiber characterized in that a plurality of cores having a high refractive index containing a rare earth element are provided, and at least one of the cores has a different shape.

According to a seventh aspect of the present invention, there is provided the rare-earth-element-doped multi-core fiber according to the second aspect, wherein the core at the center of the rare-earth-element-doped multi-core fiber has the highest value.

An eighth invention is the rare-earth-element-doped multi-core fiber according to any one of the first to seventh aspects, wherein the respective cores are in contact with each other.

A ninth invention is the rare-earth element-doped multi-core fiber according to any one of the first to seventh aspects, wherein the outer periphery of each core is coated with a cladding for coating having a low refractive index.

A tenth aspect of the present invention is the rare earth element-doped multi-core fiber according to any one of the first to ninth aspects, wherein the cladding and the core are tapered from the input side to the output side so as to have a large diameter. .

According to an eleventh aspect of the present invention, there is provided a rare-earth element-doped multi-core fiber according to any one of the first to ninth aspects, wherein the outer diameter of the multi-core fiber is reduced from a large diameter to a tapered shape along the longitudinal direction, and This is a rare-earth-element-doped multi-core fiber that is formed so as to be tapered again.

According to a twelfth aspect, the input side is the rare earth element-doped multi-core fiber according to any one of the first to ninth aspects, and each core is provided in each circular clad as it proceeds from the input side to the output side. The structure converted into a structure has one input side, and the output side has one input N output (N
> 2).

According to a thirteenth aspect, an optical demultiplexer is connected to the input side of the rare earth element-doped multi-core fiber according to any one of the first to eleventh aspects, so that signal light and pump light are propagated into the fiber.
An optical fiber amplifier for optical distribution, characterized in that an optical star coupler is connected to an output side to distribute signal light.

[0024]

When the number of cores provided is N as in the first invention, the number of cores in the clad is increased N times as compared with the conventional case. Therefore, the power of the signal light is + 10d
Even if it becomes Bm or more, the amplification degree does not decrease and a large amplified optical power can be obtained at the output side. In addition, the configuration is such that the addition amount of the rare earth element in the core at the center of the multicore assembly composed of N is the largest, and the addition amount of the rare earth element is reduced as the distance from the center to the outer periphery of the assembly increases. For example, the amount of the rare earth element added is the largest in the region where the optical power of the pump light propagating in the optical fiber is maximum, and the amount of the rare earth element is small in the region where the optical power of the pump light is weak. It contributes to the amplifier, and as a result, high power amplification can be realized.

As in the second invention, at least one of the cores is characterized by having a different refractive index. Therefore, for example, as in the seventh invention, the core is composed of N pieces. By increasing the refractive index of the core at the center in the multi-core assembly that has been made the highest, and slightly lowering the refractive index of the core as the distance from the center to the outer periphery of the assembly increases, Can be strengthened. Thus, the pump light efficiently contributes to the optical amplifier, and high power amplification can be realized.

As in the third aspect of the present invention, by covering the outer periphery of the N-piece multi-core assembly with a ring-shaped low refractive index layer having a value lower than the refractive index of the cladding, the excitation light and the signal light are separated. It can be more strongly confined within the multi-core, and higher power amplification can be realized.

According to the fourth and fifth aspects, the confinement of the pump light and the signal light in the multi-core is further increased, and the distribution of the added amount of the rare-earth element according to the light power distribution of the pump light is provided. Thereby, the excitation light efficiency can be greatly improved.

According to the sixth aspect, at least one of the cores in the multi-core aggregate composed of N pieces
One is that the shape is different, for example, if the core diameter of the center of the multi-core assembly is the largest and a core with a small core diameter is arranged on the outer periphery, the multi-core assembly is almost densely packed. A circular multi-core assembly can be formed. That is, by greatly reducing the gap between the cores, the pump light and the signal light can be efficiently confined and propagated in the respective cores.

According to the eighth aspect, the signal light and the pump light can be efficiently confined and propagated in the core by arranging the respective cores in the multi-core aggregate composed of N pieces in contact with each other. In addition, it is possible to reduce the propagation loss of the signal light, efficiently convert the energy of the pump light, and contribute to the high power amplification of the signal light.

According to the ninth aspect, by coating the outer periphery of each core with a cladding for coating having a low refractive index,
Light confinement in each core is enhanced, and scattering loss due to irregularity in the interface between the cladding for cladding and the cladding can be reduced.

According to the tenth aspect, the light reflected from the output side is reflected on the input side by forming the tapered rare earth element-doped multi-core fiber having a structure in which the crater diameter is tapered from the input side to the output side. You can suppress returning. That is, since it has an optical isolator function,
This is a very advantageous fiber structure for constituting an optical fiber amplifier.

According to the eleventh aspect, the optical fiber narrows in a tapered shape from a large diameter along the longitudinal direction, and is again tapered to a large diameter through a small-diameter portion. Of the optical star coupler can be configured. Moreover, by propagating the excitation light, an optical amplification function can be provided.

According to the twelfth aspect, it is possible to realize an optical star coupler having a 1-input N-output optical amplification function.

According to the thirteenth aspect, a system for distributing power of a signal light amplified to a high gain to many terminals can be realized with a more economical and simple configuration.

[0035]

FIG. 1 shows a first embodiment of the present invention. This is because a core having a refractive index n _w (n _w > n _C1 ) containing a rare earth element in the cladding 3 having a low refractive index n _C1 is 2-1.
9 shows a rare-earth-element-doped multi-core fiber 1 in which nine fibers are arranged as shown in FIGS. FIG. 2A is a front view of the fiber 1 and FIG. 2B is a side view, that is, an end view of the fiber 1.

As rare earth elements, besides Er (erbium), Nd (neodymium), Pr (praseodymium), Sm
(Samarium), Tm (thulium), Yb (ytterbium), Ce (cerium), Ho (holmium) and the like can be used. The core 2 is made of a glass mainly composed of SiO ₂ and contains at least one of P, Ge, Al, Cr and the like as a refractive index control additive in addition to the rare earth element. The clad 3 is made of SiO ₂ or SiO ₂ containing at least one additive for controlling the refractive index, such as F, B, or P. Here, the relative refractive index difference Δ (= (n _w −
n _c1 ) ÷ n _w × 100%) is 0. It is selected from a range of several percent to several percent.

The shape of the core 2 may be any of a circular shape, an elliptical shape, and a polygonal shape. 1μ diameter
Although a range of m to several μm is preferable, Δ and a diameter of each core 2 are selected so as to satisfy a single mode transmission condition. That is,

[0038]

(Equation 1)

Here, the values of a, n _w and Δ are selected so as to satisfy λ: the wavelength of the signal light to be transmitted and a: the diameter of the core 2.

Further, the rare earth element-doped multi-core fiber 1 of FIG. 1 can be manufactured as follows. That is, it can be obtained by bundling a number of conventional rare earth element-doped fibers into a quartz glass tube, melting the above quartz glass tube or a previously fused quartz glass tube with a heating source and drawing. This step requires at least one
The rare earth element in the core 2 is contained in a state closer to the atomic state as the number of times is increased. This is caused by the concentration quenching when a large amount of the rare earth element, which has conventionally been a problem, is added. This leads to a feature that the phenomenon of a decrease in amplification degree can be avoided. The claddings of the multiple rare earth element-doped fibers are fused together to form an integrated cladding 3. Here, each core 2-1 to 2-9
The amount of the rare earth element in the core may be different, and only one, for example, the addition amount in the core 2-1 is the largest, and the addition amount in the cores 2-2 to 2-9 is the same. You may make it less than it. As a specific example, SiO ₂ is used for the cladding 3 and SiO ₂ is used for the cores 2-1 to 2-9.
₂ -P ₂ O ₅ -Al ₂ O ₅ in the glass using 21 pieces of Er ions as the quantity of added, the addition amount of the rare earth element in the core 2-1 of the center and the maximum, of rare earth elements with distance to the peripheral portion 14 If the amount of addition is gradually reduced, when the pumping light propagates through the optical fiber 1, the structure becomes close to the rare earth element addition distribution according to the optical power distribution of the pumping light. Energy conversion can be performed well, and a high gain optical amplifier can be realized. In addition, even if the signal light is increased, the amount of the rare earth element in each core is such that the concentration quenching does not occur, so that the amplified signal light hardly saturates. That is, high power amplification can be realized. In FIGS. 1 and 2, if the refractive indices in the respective cores are made different and the pumping light and the signal light are selected so as to be concentrated at the center of the optical fiber 1, the confinement of the light becomes better and the large power Amplification can be achieved.

FIG. 3 shows a third embodiment of the multi-core fiber doped with a rare earth element according to the present invention. This is M
A ring-shaped low-refractive-index layer 4 having a refractive index n _C2 (n _C2 <n _C1 ) lower than the refractive index n _C1 of the cladding 3 is provided in the peripheral portion 14 of the multi-core aggregate. By providing the low refractive index layer 4, the confinement of the pump light and the signal light in the multi-core assembly is improved, and higher power amplification can be realized. Also in this case, if the addition amount of the rare earth element and the value of the refractive index in each core are selected so as to correspond to the optical power distribution of the excitation light and the signal light,
Further, high power amplification can be realized.

FIG. 4 shows a fourth embodiment of the present invention. This is an embodiment in which the shapes of the cores in the multicore assembly composed of 13 pieces are different. In this embodiment, the core is set to pm and the addition amount of Er ions in the cores 2-2 to 2-9 is set to 400 ppm. As shown in FIG. 2, the configuration of the core 2-1 at the center of the core to which the rare earth element is added is maximized, as shown in FIG. The shape of the cores 2-2 to 2-13 around the periphery is selected to be small. By adopting such a configuration, the outer peripheral portion 14 of the multi-core aggregate can be made to have a substantially circular shape, the polarization dependence is small, and the propagation loss can be reduced. Also in this configuration, the amount of the rare earth element added in each core and the value of the refractive index may be different. Further, a low refractive index layer 4 may be provided on the outer peripheral portion 14 as shown in FIG.

FIG. 5 shows a fifth embodiment of the present invention. This is a configuration in which 13 cores 2-1 to 2-13 are united together. With this united structure, the signal light and the pumping light can be almost propagated in the united core. Therefore, the cladding that has been interposed between the cores is eliminated, and unnecessary scattering in the cladding that is interposed between the cores is eliminated, and the signal light can be propagated with low loss. Also, the pump light is efficiently converted into energy, which contributes to high power amplification of the signal light. In addition,
In FIG. 5, the shape of each core changes according to the number of cores. That is, as the number of cores increases, the shape of the individual cores when they are combined becomes closer to a circle. In addition, as the number of cores increases, the amount of rare earth element added to each core can be suppressed to a range where concentration quenching does not occur, so that high power amplification becomes easier. FIG. 6 shows another embodiment of the rare earth element-doped multi-core fiber of the present invention. This is a structure in which the rare earth element-added cores 2 are dispersed and arranged almost all over the cladding 3. For this purpose, in addition to the high power amplification, an optical fiber coupler which is inserted between N input and output fibers and which is suitably added to realize an N-input N-output high power amplification optical star coupler is used. The multi-core fiber has a structure in which the amount of Er ions in the core 2-1 is 1000 p.

FIG. 7 also shows another embodiment of the multi-core fiber doped with a rare earth element according to the present invention. This is intended to enhance the confinement of light in the core 2 and to reduce scattering loss due to irregularity in the interface between the cladding 4 and the cladding 3.

FIG. 8 shows a fifth embodiment of the present invention.

In this embodiment, the core 2 and the clad 3 are
As shown in FIG. 8A, a tapered rare earth element-doped multicore fiber 5 having a structure in which the diameter is increased from the input side 6 shown in FIG. 8B to the output side 7 shown in FIG. Things. This suppresses the light reflected from the output side 7 from returning to the input side. That is, since it has an optical isolator function, it is a very advantageous fiber structure for constituting an optical fiber amplifier.

FIG. 9 shows a system application example of the rare-earth-element-doped multi-core fiber 1.

This is to amplify the power of the signal light indicated by the arrow 8 incident on the rare-earth element-doped multi-core fiber 1 via the optical demultiplexer 10 and to convert the amplified signal light to 1-input M output 1
The power is distributed by the × M-type optical star coupler 11, and the distributed light is output from the respective output sides as indicated by arrows 12-1, 12-2,..., 12-M.

In this case, for example, a multi-core fiber obtained by adding Er to the rare-earth-element-doped multi-core fiber 1 is used, and the signal light incident from the arrow 8 uses a 1.5 μm band. A wavelength of 1.48 μm (or 0.98 μm) is used for the excitation light indicated by the arrow 9. The optical demultiplexer 10 allows the signal light to pass through as it is, demultiplexes the pump light, and makes any light enter the rare-earth element-doped multi-core fiber 1 and propagate. Then, the signal light is power-amplified and made incident into the 1 × M-type optical star coupler 11. The 1 × M optical star coupler 11 divides the power-amplified signal light into M equal parts, and outputs an arrow, 12-1, 12-2,
The output is distributed as 12-M. This is effective as a system for distributing the amplified signal light to a large number of terminals as described above.

FIG. 10 also shows a system application example, which is an effective configuration as a power distribution system having an optical isolator function using a tapered rare earth element-doped multi-core fiber 5. With this configuration, for example, the reflected light from the distributed light extraction end side is radiated to other than the core 2 by propagating in the tapered rare earth element-doped multi-core fiber 5 and does not return to the input end side. Configuration.

9 and 10, the optical demultiplexer 1
The 0 and 1 × M-type optical star couplers 11 may have an optical fiber structure or an optical waveguide structure.

9 and 10, between the rare-earth-element-doped multi-core fiber 1 (or the tapered rare-earth-element multi-core fiber 5) and the 1 × M-type optical star coupler 11, shown in FIGS. When the optical isolator 13 is inserted as described above, more stable power amplification can be realized.

In FIG. 9 and FIG. 10, the optical demultiplexer 10 is inserted between the rare earth element-doped multi-core fiber 1 (or the tapered rare earth element-doped multi-core fiber 5) and the 1 × M-type optical star coupler. By removing the pumping light, an optical amplifier with lower noise can be realized. Further, in the above configuration, the position where the pumping light is made to enter is so-called forward pumping (FIG. 9) in the same direction as the signal light propagation direction, and so-called backward pumping (not shown) in the direction opposite to the signal light propagation direction. either will do.

FIG. 11 shows an embodiment of a 1-input N-output (N: 9) optical amplification type coupler for optical distribution according to the present invention. This is a configuration in which the input side of the rare-earth-element-doped fiber bundle inserted into the glass tube 14 is tapered and the input side is a rare-earth-element-doped multi-core fiber, and the output side is not drawn. FIG. 2A is a front view thereof,
(B) is a side view on the input side, and (c) is a side view on the output side. When a normal optical fiber (for example, a single mode optical fiber having a core diameter of 10 μm and a cladding diameter of 125 μm) is connected to the input side and signal light and pump light are propagated from the input side optical fiber, the output side rare earth element-doped fiber 15 High power optical signals amplified from -1 to 15-9 are respectively extracted. Here, the diameter of the core 2 and the diameter of the cladding 3 of the rare-earth element-doped fibers 15-1 to 15-9 on the output side are selected so as to be easily connected to an ordinary optical fiber. Also, the diameter of the aggregate of the cores 2 of the input-side rare-earth element-doped multi-core fiber is selected to a value corresponding to the core diameter of the ordinary optical fiber.

The present invention is not limited to the above embodiment. First, the dimensions and materials of the rare-earth element-doped multi-core fiber are not particularly limited, and can be selected from a wide range. For example, the fiber may be for multi-mode transmission having a V value of 2.405 or more, in addition to for single-mode transmission. The material may be borosilicate-based, fluoride-based, phosphate-based, alkali-based glass or the like in addition to the SiO ₂ -based material.

[0056]

The present invention has the following effects.

Assuming that the number of cores provided is N as in the first invention, the number of cores in the cladding is increased N times as compared with the conventional case. Therefore, the power of the signal light is +1
Even if it becomes 0 dBm or more, the amplification degree does not decrease and large amplified optical power can be obtained at the output side. And N
If the configuration is such that the addition amount of the rare earth element in the core at the center of the multi-core assembly composed of individual pieces is the largest, and the addition amount of the rare earth element is reduced as the distance from the center to the outer periphery of the assembly increases, The region where the optical power of the pumping light propagating in the fiber is the highest, the amount of rare earth element added is the largest in the region where the optical power of the pumping light is weak, and the amount of the rare earth element added is small in the region where the optical power of the pumping light is weak. This contributes to high power amplification.

As described in the second invention, at least one of the cores has a different refractive index. For example, as in the seventh invention, N cores are used. By increasing the refractive index of the core in the center of the configured multi-core aggregate and by slightly lowering the refractive index of the core away from the center toward the outer periphery of the aggregate, the multi-core of the excitation light and the signal light is increased. The confinement inside can be strengthened. As a result, the pump light efficiently contributes to the optical amplifier, and high power amplification can be realized.

As in the third aspect of the present invention, the outer periphery of a multi-core assembly composed of N pieces is covered with a ring-shaped low refractive index layer having a value lower than the refractive index of the cladding, so that the pumping light and the signal light are covered. It can be more strongly confined within the multi-core, and higher power amplification can be realized.

According to the fourth and fifth inventions, the confinement of the pump light and the signal light in the multi-core is further increased,
In addition, by providing a rare earth element addition distribution according to the optical power distribution of the excitation light, the excitation efficiency can be greatly improved.

According to the sixth aspect, at least one of the cores in the multi-core aggregate composed of N pieces
One is that the shape is different, for example, if the core diameter of the center of the multi-core assembly is the largest and a core with a small core diameter is arranged on the outer periphery, the multi-core assembly is almost densely packed. A circular multi-core assembly can be formed. That is, by greatly reducing the gap between the cores, the pump light and the signal light can be efficiently confined and propagated in the respective cores.

According to the eighth aspect of the present invention, the signal light and the pump light can be efficiently confined and propagated in the core by arranging the respective cores in the multi-core aggregate composed of N pieces in contact with each other. In addition, it is possible to reduce the propagation loss of the signal light, efficiently convert the energy of the pump light, and contribute to the high power amplification of the signal light.

According to the ninth aspect, by coating the outer periphery of each core with a cladding for coating having a low refractive index,
Light confinement in each core is enhanced, and scattering loss due to irregularity in the interface between the cladding for cladding and the cladding can be reduced.

According to the tenth aspect of the present invention, the light reflected from the output side is reflected on the input side by forming the tapered rare earth element-doped multi-core fiber having a tapered diameter from the input side to the output side. You can suppress returning. That is, since it has an optical isolator function,
This is a very advantageous fiber structure for constituting an optical fiber amplifier.

According to the eleventh aspect, the optical fiber is tapered from a large diameter to a tapered shape along the longitudinal direction of the optical fiber, and is again tapered to a large diameter through a small diameter portion. Of the optical star coupler can be configured. Moreover, by propagating the excitation light, an optical amplification function can be provided.

According to the twelfth aspect, an optical star coupler having an optical amplification function of 1 input and N outputs can be realized.

According to the thirteenth aspect, a system for distributing the power of the signal light amplified to a high gain to many terminals can be realized with a more economical and simple configuration.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an embodiment of a rare earth element-doped multi-core fiber according to the present invention.

FIG. 2 is an explanatory view of an embodiment of the rare earth element-doped multi-core fiber of the present invention.

FIG. 3 is an explanatory view of an embodiment of the rare earth element-doped multi-core fiber of the present invention.

FIG. 4 is an explanatory view of an embodiment of the rare earth element-doped multi-core fiber of the present invention.

FIG. 5 is an explanatory view of an embodiment of the rare earth element-doped multi-core fiber of the present invention.

FIG. 6 is an explanatory view of an embodiment of the rare earth element-doped multi-core fiber of the present invention.

FIG. 7 is an explanatory view of an embodiment of the rare earth element-doped multi-core fiber of the present invention.

FIG. 8 is an explanatory view of an embodiment of the tapered rare earth element-doped multi-core fiber of the present invention.

FIG. 9 is an explanatory diagram of an embodiment of application of the system of the present invention.

FIG. 10 is an explanatory view of an embodiment of application of the system of the present invention.

FIG. 11 is an explanatory view of an embodiment of application of the system of the present invention.

FIG. 12 is an explanatory diagram showing a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Rare earth element doped multi-core fiber 2 Core 3 Cladding 4 Low refractive index layer

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical indication // C03B 37/012 C03B 37/012 A H01S 3/094 H01S 3/094 S

Claims (13)

(57) [Claims] A plurality of high-refractive-index cores containing a rare-earth element are provided in a low-refractive-index cladding, and at least one of the cores has a different amount of the rare-earth element added. Characterized rare earth element-doped multi-core fiber. 2. A method according to claim 1, wherein a plurality of cores having a high refractive index containing a rare earth element are provided in a cladding having a low refractive index, and at least one of the cores has a different refractive index value. Rare earth element doped multi-core fiber. 3. A plurality of high-refractive-index cores containing a rare-earth element are concentrated and distributed in the vicinity of a central portion of the clad in the low-refractive-index cladding, and the outer periphery of the region where the cores are concentrated is provided. Characterized in that a ring-shaped low refractive index layer having a value lower than the refractive index is provided. 4. The rare-earth element-doped multi-core fiber according to claim 3, wherein at least one of the cores has a different amount of rare-earth element added. 5. The rare-earth element-doped multi-core fiber according to claim 3, wherein at least one of the cores has a different refractive index value. 6. A plurality of high refractive index cores containing a rare earth element are provided in a low refractive index cladding, and at least one of the cores has a different shape. Rare earth doped multi-core fiber. 7. The rare earth element-doped multi-core fiber according to claim 2, wherein the refractive index of the core at the center of the rare earth element-doped multi-core fiber has the highest value. 8. The rare-earth element-doped multi-core fiber according to claim 1, wherein the respective cores are in contact with each other. 9. The rare-earth element-doped multi-core fiber according to claim 1, wherein the outer periphery of each core is coated with a cladding for coating having a low refractive index. 10. The rare-earth element-doped multi-core fiber according to claim 1, wherein the cladding and the core are tapered from the input side to the output side so as to have a large diameter. 11. A structure in which the outer diameter of a rare-earth element-doped multi-core fiber is tapered from a large diameter to a tapered shape along a longitudinal direction, and is again tapered through a small-diameter portion coupled with a propagation mode. The rare-earth element-doped multi-core fiber according to any one of claims 1 to 9. 12. The input side is the rare earth element-doped multi-core fiber according to any one of claims 1 to 9, wherein each core is provided in a circular clad as going from the input side to the output side. A one-input N-output (N> 2) coupler for light distribution, comprising one input side and a multi-core rare-earth element-doped fiber in the converted structure. 13. An optical demultiplexer is connected to the input side of the rare-earth element-doped multi-core fiber according to any one of claims 1 to 11, to propagate signal light and pump light into the fiber, and to the output side. An optical fiber amplifier for optical distribution, wherein an optical star coupler is connected to distribute signal light.